Fix silent column collisions and accounting-style negatives in ingest

[shivamlalakiya]

What

Two bugs in the ingest helpers:

1. map_columns let a mapping send two different source columns to the
   same target name. The rename went through silently and produced a
   DataFrame with two identically named output columns, which could
   still pass a required= check because the column name existed, just
   twice. It now raises a ValueError naming the colliding source
   columns before the rename happens.

2. _to_amount (shared by the CiviCRM, Raiser's Edge, and NPSP gift
   readers, since the latter two build on civicrm_contributions_to_features
   rather than reimplementing amount parsing) stripped parentheses along
   with every other non-numeric character, so an accounting-style
   negative like "($50.00)" parsed as positive 50 instead of -50. It
   now converts a parenthesised amount to a leading minus sign first.

Why

Both are silent-corruption bugs: a mis-mapped column passes validation
and a refund or credit note gets summed into a donor's lifetime giving
as a positive gift.

How tested

Added a regression test for each bug and confirmed both failed before
the fix:

• tests/test_map_columns.py::test_raises_on_collision_naming_the_colliding_sources
• tests/test_civicrm.py::test_to_amount_parses_signed_and_formatted_values
  (parametrized over "($50.00)", "(50)", "-25.50", "$1,000.00")

Ran the full targeted suite for the touched readers to confirm nothing
else changed meaning:

python -m pytest tests/test_map_columns.py tests/test_civicrm.py tests/test_raisers_edge.py tests/test_npsp.py
163 passed

make ci and make riskcov both pass locally (2200 passed, 30 skipped,
98.35% coverage against a 92% floor; 98% risk-tier coverage against a
93% floor).

CHANGELOG.md updated under [Unreleased].